Electric drive system with an electronically commuted DC motor in order to reduce torque irregularities

ABSTRACT

The invention relates to an electric drive device with a DC motor ( 2 ) comprising a control circuit with an electronic commutator ( 3 ). The invention is characterized in that a derivation of a control signal (V_i_ref) is obtained from an induced motor voltage (E_sample) detected by a measuring device and from a reference value (V_i_av) which serves to regulate the speed of the DC motor ( 2 ), and in that the derived control signal (V_i_ref) serves to achieve a substantially constant torque of the DC motor ( 2 ) through adjustment of the motor currents (ia, ib, ic).

The invention relates to an electric drive device with a DC motor, inparticular with a permanent-magnetically excited air core coil motor,with a control circuit comprising an electronic commutator.

Electronically commutated, permanent-magnetically excited small DCmotors of low inductance, such as air core coil motors, satisfy highrequirements as regards quiet running. This property becomesincreasingly important for spindle drives in computer drive units,because the achievable storage densities increase. Air core coil motorsare particularly suitable here, because they generate no interferingdetent torques and radial forces.

Known spindle drives are fitted exclusively with slotted-core ironarmature motors. There are suggestions for hard disk drives with aircore coil motors (foil motors) with internal rotor as described, forexample, in U.S. Pat. No. 5,714,828. A new development in the field ofelectronic commutation of motors for hard disk drives is “phase currentshaping”. Here the phase current is modulated in the three individualphases with the object of achieving a certain—for example approximatelysinusoidal—gradient of the phase currents. Usually the motor voltage ispulse width modulated (PWM) in this case, because otherwise theswitching losses would be very high. The PWM duty cycle serves as themanipulated variable. This method as a rule requires a PLL-supportedcommutation control on account of the more difficult zero passagerecognition of the induced voltage. Furthermore, such a phase currentshaping cannot be transferred to air core coil motors (for example withfoil windings) without modifications. These motors indeed haveconsiderably smaller electric time constants compared with the ironarmature types (i.e. approximately one tenth) and require acorrespondingly higher pulse frequency of the PWM, which in its turn canbe realized with additional expenditure only.

EP 0773624 discloses another method of reducing the torque ripplethrough modification of the current in the intermediate circuit only,while retaining the conventional 120° square wave commutation,preferably by sensorless commutation (EMF commutation). Here the motorvoltage is raised after each commutation moment in order to achieve asomewhat more even intermediate circuit current and in addition to avoidtorque glitches after switching caused by the operation and theinductance. The signal for this is obtained from the discharging processof an RC member. The torque ripple can indeed be reduced in this manner,but only to a certain degree, because it is to be observed that theproduct of current and flux linking change is to be kept constant at alltimes. This, however, requires a continuous adaptation of the current.This method is not very suitable for motors with a very small electrictime constant.

It is an object of the invention to reduce commutation-dependent torquefluctuations of permanent-magnetically excited motors with air corecoils.

According to the invention, this object is achieved in that a derivationof a control signal is obtained from an induced motor voltage detectedby a measuring device and from a reference value which serves toregulate the speed of the DC motor, and in that the derived controlsignal serves to achieve a substantially constant torque of the DC motorthrough adjustment of the motor currents (ia, ib, ic). This motorcontrol reduces the commutation-dependent torque ripple oflow-inductance small DC motors. For this purpose, the 120° square wavecommutation with zero passage detection is retained, and theintermediate circuit current only is modulated, so that a modulation ofthe single phase currents is omitted and a conventional commutationmethod, such as sensorless commutation (EMF commutation) can be used.The signal necessary for this is obtained in a simple manner from theinduced voltage. The circuitry expenditure is clearly reduced incomparison with other methods thereby.

The embodiment as claimed in claim 2 renders possible a simpledetermination of the reference value for the motor current at which themotor torque is constant. The embodiment of claim 3 renders possible adetermination of the reference value for the motor current at which themotor torque is constant, also if the motor speed is not constant.

The embodiment of claim 4 renders possible a simple adjustment of theintermediate circuit current by means of a longitudinal control and acontrol member, using the control signal.

In the embodiment of claim 5, the control member may be omitted in thatan inverter belonging to the commutator is directly controlled by thecontrol unit.

The invention will be explained in more detail below with reference to anumber of Figures, in which

FIG. 1 is a block diagram showing the circuit of an electric drivedevice according to the invention,

FIG. 2 shows the arrangement of the reference value determination forthe motor current in block 4 in more detail,

FIG. 3a shows the principle and FIG. 3b a practical implementation ofthe signal processing which takes place in block 6, and

FIGS. 4a to 4 h show a number of signal gradients in the motor control.

The block diagram of FIG. 1 shows the entire drive comprising aninverter 1, a DC motor 2, an EMF commutator 3, and a block 4 forobtaining the reference signal and a block 5 which converts thisreference value for the intermediate circuit current by means of anadjustment step. The EMF commutator 3 operates with 120° phase shifts inthe square wave mode and with zero passage detection, the abbreviationEMF (electro-motive-force) denoting a sensorless commutation, whichmeasures the induced motor voltage in the respective currentless statesof the three phases Ea, Eb, Ec. The exclusive use of an EMF commutator3, however, leads to problems with the torque ripple in the case ofmotors 2 of low inductance, in particular air core coil motors, which iswhy a block 4 with a novel circuit is added.

FIG. 2 shows the construction of the block 4 for forming the referencevalue in detail. Its input quantities are on the one hand thecommutation signal V_FG, pictured in FIG. 4b, the measured inducedvoltage E_sample, shown in FIG. 4a, where φ denotes the electrical angleof rotation of the motor 2. These two quantities are supplied by the EMFcommutator. Furthermore, a control signal V_i_av is applied by a controlcircuit (not shown), by means of which the speed of rotation of themotor 2 can be set.

The signal V_i_av is the reference value for the motor current valueaveraged in time generated in a higher order speed control. Since thetwo signals V_FG and E_sample are available in the EMF commutator 3anyway, only one new, additional block 4 is necessary for minimizing thetorque ripple of the motor 2. The reference value for the instantaneousmotor current is obtained in block 4, as shown in FIG. 2, in that firstE_sample is inverted in each second commutation cycle, and the signalE_sample2 thus generated is integrated, which leads to the signal dFlux.A filter may be connected between the inversion step and the integrationstep, as shown in FIG. 2, so as to filter out any DC component which maybe present from the signal E_sample2. The gradient of dFlux is shown inFIG. 4d. The signal dFlux is then passed on to block 6, whose operationwill be explained below.

The reference value V_i_ref for the instantaneous value of the motorcurrent, which achieves a constant motor torque, can be obtained fromthe signal dFlux by means of the relation indicated in FIG. 3a. Thesignal Flux of FIG. 4e is obtained by means of the relation 1+c1*dFlux.A standardizing factor c1 is set for this purpose such that a ratio ofthe maximum value to the minimum value of the signal Flux results whichcorresponds to the ratio of the maximum value to the minimum value ofthe rectified induced voltage. In the case of an ideal three-phasemotor, this ratio is equal to the factor 2/3.

In practice, the relation of FIG. 3 can be approximatively evaluated ina very simple manner with a circuit 6 as shown in FIG. 3b in that thesignal dFlux, after being amplified by a constant factor c2 which is tobe set, is subtracted from the reference signal for the instantaneousaverage value of the motor current V_i_av, which reference signal is tobe regarded as constant here. This functions highly satisfactorily formotors 2 which have to cover only a narrow range of speeds such as, forexample, drives for hard disk units which operate at a constant speed,because the reference signal V_i_av is constant here because of theconstant speed of rotation, as is the factor c. The signal V_i_ref thuscreated is pictured in FIG. 4g. The factor c1 must be adapted to theinstantaneous speed of rotation in the case of a drive with variablespeed, which is possible, however, by means of an arithmetic unit withthe use of the speed-dependent signal V_FG.

The algorithm described in FIGS. 2 and 3 may be realized by means ofanalog or digital signal processing units.

The signal V_i_ref is the controlling quantity for the current i_dc ofan intermediate circuit control 8 which can be adjusted by means of acomparison between a target value and an actual value with a subsequentcontrol operation by a control member 5. This may be achieved, forexample, by means of a longitudinal control 8. Then the intermediatecircuit current i_dc is commutated to the three phases by the inverter1.

Alternatively, the output of the control 8 may be utilized as acontrolling quantity for a joint adjustment operation for the invertertransistors 1, which is shown with a broken line in FIG. 1. In thiscase, the supply voltage v_bat and the intermediate circuit voltage v_dcare identical, because the control member 5 is absent in thisembodiment. The control of the inverter 1, on the other hand, is morecomplicated here, because it carries out not only the commutation, butalso has to control the torque ripple, for which a coordination of thetwo processes in one electronic circuit 7 is necessary. FIG. 4h shows byway of example the motor current ia in a phase Ea at a minimum torqueripple, while the other motor currents have respective phase shifts of120°.

What is claimed is:
 1. An electric drive device with a DC motor with acontrol circuit comprising an electronic commutator, wherein aderivation of a control signal is obtained from an induced motor voltagedetected by a measuring device and from a reference value which servesto regulate the speed of the DC motor, and wherein the derived controlsignal serves to achieve a substantially constant torque of the DC motorthrough adjustment of the motor currents, and wherein an inversion ofthe measured induced voltage of the commutator is provided in everysecond commutation cycle, followed by an integration and a derivationtherefrom of a signal, which after amplification and subsequentsubtraction from the reference value is available as a control signal.2. An electric drive device with a DC motor with a control circuitcomprising an electronic commutator, wherein a derivation of a controlsignal is obtained from an induced motor voltage detected by a measuringdevice and from a reference value which serves to regulate the speed ofthe DC motor, and wherein the derived control signal serves to achieve asubstantially constant torque of the DC motor through adjustment of themotor currents and wherein an inversion of the measured induced voltageof the commutator is provided in every second commutation cycle,followed by an integration and a derivation therefrom of a signal, whichmultiplied by a factor and after addition of 1 yields a signal by whichthe reference value is divided, in that the control signal being theresult of said division is available, and in that it is provided thatthe factor is applied by means of an arithmetic unit such that the ratioof the minimum value to the maximum value of the signal corresponds tothe ratio of the minimum value to the maximum value of the rectifiedinduced voltage.
 3. An electric drive device as claimed in claim 1,characterized in that the derived control signal is provided fortriggering a controller, and said controller is provided for controllingan intermediate circuit current by means of a control member.
 4. Anelectric drive device as claimed in claim 1, characterized in that thederived control signal is provided for triggering a controller whichcarries out a control operation on the inverter belonging to thecommutator.
 5. An electric drive device as claimed in claim 2,characterized in that the derived control signal is provided fortriggering a controller, and said controller is provided for controllingan intermediate circuit current by means of a control member.
 6. Anelectric drive device as claimed in claim 2, characterized in that thederived control signal is provided for triggering a controller whichcarries out a control operation on the inverter belonging to thecommutator.